Enhanced fatigue strength orthopaedic implant with porous coating and method of making same

ABSTRACT

A method for producing an orthopedic implant having enhanced fatigue strength. A forged implant substrate having an elongated stem is incorporated with a melting point lowering substance. Then, metal particles are sintered to the substrate, forming a porous layer on the substrate which enhances bone ingrowth or the mechanical interlock with bone cement. Advantageously, the sintering occurs at a lower temperature than if the substance were not incorporated into the substrate, which in turn results in an enhanced fatigue strength of the inventive implant. The fatigue strength of a forged or cast implant can also be improved by nitrogen diffusion hardening and/or thermally processing the implant after the porous coating is adhered by sintering. Further, the fatigue strength can be further improved by combining incorporating the melting point lowering substance with nitrogen diffusion hardening and/or aging treatment subsequent to sintering.

[0001] The present application is a divisional of Ser. No. 09/267,272filed Mar. 12, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates to prosthetic implants having aporous surface attached thereto and more particularly to improving thefatigue strength of a such an implant.

[0004] 2. Description of Related Art

[0005] Prosthetic implants for the replacement of a portion of apatient's joints are well-known, and may be constructed ofcobalt-chromium-molybdenum or titanium, for example. Similarly, it isknown to provide a porous surface layer on the implant to promotefixation by allowing direct bone ingrowth and interdigitation with theimplant. Alternatively, the porous surface may receive bone cementtherein to enhance the mechanical interlock with bone cement The poroussurface layer typically takes the form of a plurality of small metallicparticles such as beads or a wire mesh. Commonly, the porous surfacelayer is sintered, diffusion bonded, or welded to the implant. Theseprocesses require heating the implant and particles to a temperaturesufficient to cause the porous surface layer and implant body to fuse,melt or bond together at their point of mutual contact.

[0006] A phenomenon with beaded and/or other textured surfaces is thatthe texturing creates a “notch effect” on the surface of the implant. Ifthe bonded junctions were viewed in cross section, a small notch wouldbe seen extending into the implant on each side of a contact pointbetween the porous surface layer and the implant. This so-called “notcheffect” contributes to crack formation when the implants are cyclicallyloaded in a fatigue mode.

[0007] A related phenomenon with beaded or textured surfaces is that thesintering process by which the beads are typically adhered to theimplant creates an annealing effect which reduces the fatigue strengthof the implant. This annealing effect is particularly noticeable inforged implants which have a higher fatigue strength, due to working ofthe metal, than their cast counterparts before bead bonding.

[0008] U.S. Pat. No. 5,443,510, assigned to the assignee of the presentinvention and hereby incorporated by reference, discloses addressing the“notch effect” phenomenon by reducing the number of notches formed. Theformation of notches in the implant body can be limited by creating athin layer of metal mesh on the surface of the implant and then bondingthe porous surface layer onto the mesh.

[0009] U.S. Pat. No. 5,734,959, assigned to the assignee of the presentinvention and hereby incorporated by reference, discloses a method forenhancing the bonding between the porous surface layer and the implant.An organic binder such as a water-soluble protein is used to enhance thebonding between the porous surface layer and the implant. During thesintering process, the binder carbonizes and alloys with the metal ofthe porous surface layer and thereby lowers the melting temperature ofthe metallic particles at the interface surfaces. Other alloy materialssuch as silicon can also be suspended in the binder with this methodThis patent does not address the notch effect phenomenon.

[0010] U.S. Pat. No. 5,308,412, assigned to the assignee of the presentinvention and hereby incorporated by reference, discloses a method forsurface hardening cobalt-chromium based orthopaedic implants by anitriding or nitrogen diffusion hardening process. The '412 patent isaimed at increasing the wear-resistance properties of the surface of theimplant so as to reduce the wear debris produced from articulationagainst polyethylene, metal, or ceramic counterfaces or by micro-motionof the implant relative to the environment contacting the implant,typically bone or bone cement. The '412 patent suggests that thenitriding process disclosed therein results in minimal or no loss offatigue properties to the implant.

SUMMARY OF THE INVENTION

[0011] The present invention provides an implant having enhanced fatiguestrength by incorporating a substance into the implant which reduces themelting point of the substrate prior to sintering the porous layer tothe substrate. In so doing, sintering can be performed at a lowertemperature, which in turn significantly reduces the fatigue strengthloss from a forged implant which occurs during the sintering process.

[0012] The present invention also provides a nitriding process andthermal processes to which the implant can be subjected after thesintering process is completed The nitriding or nitrogen diffusionhardening process and the thermal processes further increase the fatiguestrength of a cast or forged implant.

[0013] In one form thereof, the present invention provides a method forforming a porous layer on a forged orthopaedic implant. First, anorthopaedic implant substrate formed from a forged metal alloy andhaving a surface adapted to support a porous layer and a plurality ofmetallic particles are provided. A substance is incorporated into theforged substrate which substance reduces the melting point of thesubstrate. The substrate surface and the metallic particles are broughtinto contact with one another and heated to a temperature less than thereduced melting point, whereby the particles bond to the surface. In apreferred form, the forged alloy is cobalt-chromium-molybdenum alloy.The melting point lowering substance can be carbon, silicon, nitrogen,niobium, columbium, tantalum, chromium carbides, chromium nitrides,chromium silicides, molybdenum silicides, chromium borides, siliconcarbides, silicon nitrides, titanium carbides, titanium aluminides,titanium silicides, zirconium carbides or zirconium silicides.

[0014] One advantage of the method described above is that itcompensates for the “notch effect” and the reduction in fatigue strengthwhich results from the high temperatures and long times involved insintering. That is, the method in accordance with the present inventionprovides a forged implant which maintains most of its fatigue strengththrough the sintering process.

[0015] In another form thereof, the present invention comprises a methodfor increasing the fatigue strength of an implant having a porous layerthereon. The implant can be formed from either a cast or forgedmaterial. An implant substrate formed from a metal alloy and having asurface adapted to support a porous layer and a plurality of metallicparticles are provided. The metallic particles are brought into contactwith the substrate surface. The metallic particles and implant substrateare heated to a temperature sufficient to sinter the particles to thesurface, whereby the particles bond to the surface and form a porouslayer Then, the implant is gas quenched down to at least roomtemperature. The substrate is then heated to an aging temperature rangeof about 800° F. to 2100° F. and aged at the aging temperature for 1 to100 hours.

[0016] In a preferred form, the method includes gas quenching theimplant to below −90° F. or between −90° F. and −300° F. during the gasquenching step.

[0017] One advantage of these thermal processing methods is that theycan be used in addition to or separately from the method ofincorporating a melting point lowering substance into the substrate.

[0018] Another advantage of the inventive thermal processing methods isthat they can be used to enhance the strength of forged or cast parts,and can be used with porous coated or uncoated implants.

[0019] In yet another form, the present invention provides a method ofincreasing the fatigue strength of a beaded implant. The implant can beforged or cast. A beaded orthopaedic implant substrate is provided andthen exposed to an atmosphere of molecular nitrogen gas or atomicnitrogen at a process temperature within the range of 500° F. to 2400°F. for a process time duration sufficient to achieve increased fatiguestrength.

[0020] An advantage of the inventive nitriding process in accordancewith the present invention is that it significantly improves the fatiguestrength of a cast or forged implant. Thus, cast or forged implantssubjected to sintering can have their fatigue strength restored bysubsequently using the nitrogen diffusion hardening or nitriding processin accordance with the present invention.

[0021] Another advantage of the inventive methods of the presentinvention is that they can improve the mechanical properties of a widevariety of implants, such as for hip, knee, shoulder, elbow and otherjoints.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above-mentioned and other features and objects of thisinvention, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

[0023]FIG. 1 is a perspective view of a hip stem having a porous coatattached thereto in accordance with the illustrated embodiment,

[0024]FIG. 2 is a cross sectional view taken along line 2-2 of FIG. 1;and

[0025]FIG. 3 is an enlarged fragmentary view illustrating the poroussurface of the hip stem of FIG. 1

[0026] Corresponding reference characters indicate corresponding partsthroughout the views. Although the drawings represent an embodiment ofthe present invention, the drawings are not necessarily to scale andcertain features may be exaggerated in order to better illustrate andexplain the present invention. The exemplification set out herein is notto be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0027] As shown in FIG. 1, an orthopaedic implant 10 in the form of hipstem 12 comprises a substrate 11 (FIG. 2) and porous layer 14. Porouslayer 14 is comprised of metallic particles. For the purposes of thisspecification, the term “particles” is to be construed broadly andincludes beads, fibers, wire mesh and other known materials and shapesthereof used to form porous layer 14. As shown enlarged in FIG. 3, theparticles in the illustrated embodiment are round beads 16. Beads 16 canbe bonded to substrate 11 by a known sintering process in which thebeads are brought into contact with the substrate and heat is applied,which causes atomic bonding of the beads to the substrate.

[0028] It has been found that the fatigue strength of forged (orwrought) cobalt-chromnium-molybdenum alloy (ASTM-F1537) implants can bebetter maintained if the beads can be bonded to the substrate attemperatures below the conventional sintering temperatures, i.e., below2385° F. The melting point of the substrate can be lowered byincorporating melting point lowering substances, such as metallic ornonmetallic elements into the substrate. Similarly, intermetalliccompounds of the same elements can be incorporated into the substrate tolower the melting point thereof It has been found that a relativelysmall reduction in melting point of, for example, only about 30-50° F.or more produces a significantly stronger substrate after porous layer14 is sintered thereto. This is so because much of the reduction infatigue strength resulting from heating does not occur until thetemperature of the substrate approaches its melting temperature. Statedanother way, if fatigue strength were plotted as a function ofincreasing temperature, the resulting graph would be a fairly horizontalline until 50-150° F. below the melting point of the substrate,whereupon a sharp descending curve would appear. Thus, it can beappreciated that relatively small reductions in melting temperature, andin turn sintering temperature, of the surface of the substrate canresult in significant preservation of fatigue strength of the implant.

[0029] As a related advantage, the time required to successfully sinterthe particles to the substrate can be reduced with these melting pointlowering substances incorporated into the substrate. The melting pointof beads 16 can also be lowered by incorporation of these metallic ornonmetallic elements and compounds into the beads.

[0030] Forming of the porous layer on an implant is generally known toone within ordinary skill in the art, and need not be discussed indetail here. Most generally, an orthopaedic implant substrate having asurface adapted to support a porous layer and a plurality of metallicbeads are provided. According to the present invention, a melting pointlowering substance is then incorporated into the surface of thesubstrate using the commercially available methods describedhereinbelow. The beads are brought into contact with the surface of thesubstrate and fused thereto by heating to a temperature at whichsintering takes place, the temperature being less than the reducedmelting point produced by incorporation of the substance. The sinteringcan be performed in a conventional sintering oven, for example, and as aresult, the metallic particles bond to the substrate.

[0031] It is to be understood that substrate 11 can be formed from anyforged cobalt-chromium-molybdenum or other cobalt base alloys.

[0032] Many different elements and intermetallic compounds can lower themelting points of the substrate These melting point lowering substancesinclude carbon, silicon, nitrogen, niobium, (or columbium), tantalum,chromium carbides, chromium nitrides, chromium silicides, molybdenumsilicides, chromium borides, silicon carbides, silicon nitrides,titanium carbides, titanium aluminides, titanium silicides, zirconiumcarbides and zirconium silicides.

[0033] Nitrogen diffusion hardening or nitriding processes involve thesurface of the substrate being alloyed with nitrogen by placing theimplants and/or beads in a gaseous environment of nitrogen, whichresults in the alloy having a reduced melting point. Through the processof nitrogen diffusion hardening, nitrides such as CrN₂, CoN₂ and MoN₂are formed in a surface layer on the substrate. The process of nitrogendiffusion hardening is well-known and is described, for example, in U.S.Pat. No. 5,308,412, assigned to the assignee of the present inventionand hereby incorporated by reference. It has been hitherto unknown touse the process of nitrogen diffusion hardening to improve the fatiguestrength of a porous coated forged implant.

[0034] An ion implantation process can be used to incorporate themelting point lowering substance into the implant substrate.Commercially available ion implantation processes typically involveextracting a stream of ions from an ion source, accelerating andfocusing them into a beam which is rastered onto the substrate.

[0035] High temperature commercially available coating processes canalso be used to coat the implant substrates and beads. Such thermalcoating processes include plasma spray coating processes, in which thesubstance to be incorporated is heated to a molten state and thendeposited onto the metal alloy, after which the substance solidifies andmechanically bonds to the substrate.

[0036] Blasting the surfaces of the implants with the substances can beaccomplished using commercially available blasting processes. Theblasting process leaves residues of the melting point loweringsubstances on the surface of the implants. As a result, the surface ofthe implants have a lower melting point so that a good metallurgicalbond is established between the implant surface and the beads at a lowersintering temperature. Incorporation by blasting process is furtheradvantageous in that the blasting “work hardens” the surface.Additionally, blasting produces a slightly abraded surface which helpsthe beads to adhere thereto.

[0037] Examples of melting point depressants that can be added to thealloy surface using the above-described processes are CoCO₃, Co₂P,CoMoO₄, CoSi, Co₂Si, CoSi₂, Co₃Si, CoS, CoS₂, etc.

[0038] Alternatively, sintering of the beads to the implant substratecan be conducted in a nitrogen or carbon atmosphere. For example, thebonding is advantageously performed in a chamber filled with nitrogengas. Preferably, the atmosphere comprises greater than 99% nitrogen.Likewise, the bonding is advantageously performed in a chamber filledwith a carbon containing gas such as carbon dioxide or methane. Inaddition, the oxygen in the atmosphere is advantageously reduced tolimit the effects of oxidation. Processing in such an enrichedenvironment results in carbon or nitrogen being absorbed into the solidalloy substrate, thereupon forming chromium, cobalt and molybdenumcarbides and nitrides. The presence of the carbides and nitrides lowersthe melting point of the surface of the substrate and consequentlylowers the temperature at which sintering of the beads to the substratetakes place.

[0039] The process of incorporating melting point lowering substancesdescribed above, by itself, minimizes the fatigue strength lost by theforged substrate during the sintering process, as can be seen withreference to Table III, below. However, it has also been found thatforged and cast parts can be subjected to nitrogen diffusion hardeningor a thermal aging process after the sintering process to add strengthto the parts, as described in detail hereinbelow. Furthermore, thesubsequent nitriding and aging can be used separately of or subsequentlyto the above-described incorporation to increase the fatigue strength ofthe porous coated implant.

[0040] Nitrogen diffusion hardening of an implant substrate to improvewear resistance properties is described in U.S. Pat. No. 5,308,412.However, the '412 patent suggests that nitrogen diffusion hardening, atbest, will not reduce the fatigue strength of an implant. Surprisingly,the inventors of the present invention have found that nitrogendiffusion hardening performed subsequent to bead bonding of a forged orcast implant actually significantly improves the fatigue strength of theimplant. It is anticipated that nitrogen diffusion could be used beforebead bonding to lower the diffusion bonding temperature (describedabove) and/or used subsequent to bead bonding to further improve fatiguestrength.

[0041] With reference to Tables I and II, below, the improved fatiguestrength of porous coated forged Cobalt-Chromium-Molybdenum subjected tonitrogen diffusion hardening can be appreciated. Table I illustratesresults derived from a control sample of a forged flat piece ofCobalt-Chromium-Molybdenum bead bonded at a reduced sinteringtemperature and fatigue tested in a cantilever manner at stress ratio,R=0.1. Even though the test samples were flat, the forging process wasperformed in accordance with the forging process used for a hip stem,for example. The bead bonding was performed in a sintering oven at 2350F for 1 hour As shown in Table I, the control samples were then cycled10 million times or until they fractured. Control specimen No. 1fractured at 55 ksi loading after 3 7 million cycles and specimen No. 2failed at 60 ksi loading after 2.6 million cycles. Specimen No 3 did notfail at 57.5 ksi. Thus, this group of samples exhibited a fatiguestrength of approximately 55 ksi.

[0042] The second set of samples, recorded in Table II, underwent anitrogen diffusion hardening process after they were bead bonded usingthe same bead bonding process as used with the controls. The nitrogendiffusion hardening was performed at 2000° F. for 2 hours, usingsubstantially the same procedure described in U.S. Pat. No. 5,308,412.Generally, the nitrogen diffusion hardening comprised exposing theimplant substrate to an atmosphere of non-diluted molecular nitrogen gasat a process temperature within the range of 500° F. to 2400° F. for0.25 to 4 hours. As shown in Table II, no fracture of the nitrogendiffusion hardened substrates occurred until loading of close to 65 ksi.Thus, the nitrogen diffusion hardening process significantly improvesthe fatigue strength of the forged beaded alloy.

[0043] In addition to improving the fatigue strength of a forged alloy,the nitrogen diffusion hardening process can improve the fatiguestrength of a cast substrate. TABLE I Fracture Spec Run Actual Y = YesNo No. Tested On Load PSI HZ Cycles N = No 1 1 Machine #3 49978.12 2010000000 N 1 2 Machine #3 55000.00 20  3670295 Y 2 1 Machine #3 52401.2320 10000000 N 2 2 Machine #3 54889.34 20 10000000 N 2 3 Machine #360016.37 20  2643135 Y 3 1 Machine #29 54991.34 20 10000000 N 3 2Machine #29 57572.42 20 10000000 N

[0044] TABLE II Fracture Spec Run Actual Y = Yes No No. Tested On LoadPSI HZ Cycles N = No 1 1 Machine #2 54998.22 30 10000000 N 1 2 Machine#2 60011.46 30 10000000 N 1 3 Machine #2 62444.36 20 10000000 N 2 1Machine #11 54995.13 30 10000000 N 2 2 Machine #11 59968.84 30 10000000N 2 3 Machine #11 62526.76 30 10000000 N 3 1 Machine #28 60049.73 3010000000 N 3 2 Machine #28 64871.24 30  1944553 Y

[0045] Improvements in fatigue strength can also be obtained by thermalprocessing after bead bonding using an aging heat treatment.

[0046] After the beads are adhered to the implant substrate bysintering, the bead coated implants are cooled from the sinteringtemperature down to approximately 2100° F. The cooling to 2100° F. canbe controlled or allowed to occur naturally in the furnace or oven. Uponreaching 2100° F., the parts are quickly gas quenched down to at leastroom temperature, or lower. Gas quenching is performed by subjecting theparts to a very cool gas, such as argon or nitrogen, as is widely knownin the art. Without wishing to be tied to any specific theory, it isthought that the temperatures obtained during sintering, approximately2385° F., result in a super saturated condition in the atomicmicrostructure of the substrate. The fast cooling by gas quenching“locks in” the atomic microstructure formed during sinteringtemperatures, and allows fine precipitates of chromium and molybdenumcarbides to form throughout the substrate upon aging at elevatedtemperatures.

[0047] Thus, after the quenching step, the beaded substrate is heatedand aged in the temperature range of about 800-2100° F. for 1 to 100hours, more preferably 1 to 40 hours. Preferably, the heating and agingtakes place in an oxygen reduced atmosphere to prevent oxidation Forexample, an atmosphere comprising a partial vacuum or an inert gas suchas argon are suitable To a certain extent, the aging time is inverselyproportional to aging temperature, so that the time required to reachoptimum fatigue strength is reduced with increasing temperature. It isthought that the carbide precipitates formed during the aging processfit within the lattice of the base alloy and increase the hardness andmechanical properties thereof.

[0048] The fatigue properties of aged high carboncobalt-chromium-molybdenum alloy forgings are given in Table III. Asshown in Table III, the fatigue strength of conventional hightemperature (greater than 2350° F.) sintered cobalt-chromium-molybdenumalloy is reported (Example 1.). The fatigue strength is increased by theabove described treatments. Reducing the sintering temperature so thatit is less than or equal to 2350° F. (Example 2.) produces a significantimprovement as was discussed in conjunction with Table I It has beenfound that aging after bead bonding (Example 3.) results in an additivefatigue strength improvement. Finally, performing the sintering at areduced temperature by incorporating a melting point lowering substanceplus nitrogen diffusion hardening (Example 4.), as was discussed inconjunction with Table II, also produces an additive effect. It isbelieved that any of these processes can be used separately or incombination to improve the fatigue strength ofcobalt-chromium-molybdenum alloy implants and that when used incombination they will have an additive effect.

[0049] It can be appreciated that the aging process can be useful forapplications such as dental implants in addition to orthopaedic implantsto improve the fatigue strength thereof Additionally, the aging processcan be used for both cast as well as forged alloys. TABLE III FatigueProperties of Bead Coated High Carbon Co—Cr—Mo Alloy Forgings. FatigueStrength (ksi) R = 0.1; Process 10,000,000 cycles 1 Conventional HighTemperature Sintering Process 40.0 (Sintering temperature approximately2385° F.) 2 Reduced Temperature Sintering Process 55.0 (Sinteringtemperature = 2350° F.) 3 Reduced Temperature Process Plus Aging 60.0(Sintering temperature = 2350° F. and aged at 1400° F. for 2-10 hours) 4Reduced Temperature Sintering Process plus 65.0 Nitriding or NitrogenDiffusion Hardening (Sintering temperature = 2350° F. and nitrogendiffusion hardened in ≧ 99% N₂ at 2000° F. for 0.5-4 hours)

[0050] It has been found that the aging process produces better fatiguestrength if the implant substrate is cooled to a cryogenic temperatureof between −90° F. and −300° F. (instead of room temperature) during thequenching step. While the exact mechanism by which this cryogenictreatment operates is not understood, it is believed that cooling thesubstrate to cryogenic temperatures better preserves the super saturatedatomic microstructure formed at sintering temperatures.

[0051] Specific commercially available alloys that can be used in theaging process, with or without cooling to cryogenic temperatures,include Carpenter Biodur CCM Plus alloy (commercially available fromCarpenter Steels of Reading, Pa.), Firth Rixson high carbon alloy(commercially available from Firth Rixson Superalloys Ltd., Derbyshire,England), Teledyne Allvac high carbon alloy (commercially available fromTeledyne Allvac or Monroe, N.C.), ASTM F-75, ASTM F-799 and ASTM F-1537.

[0052] Hardening and increased fatigue strength of the alloy can also beachieved by slow furnace cooling the sintered Cobalt-Chromium-Molybdenumalloy from sintering or solution treating temperatures. The processparameters are not critical. Cooling in the furnace from the sinteringtemperature to room temperature over a period of greater than one-halfhour produces the desired result. During the slow furnace coolingprocess, chromium carbides will precipitate in the atomic microstructurelattice and harden the alloy.

[0053] While this invention has been described as having an exemplarydesign, the present invention can be further modified within the spiritand scope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. An orthopaedic implant comprising a body having an outer surface, theouter surface including a melting point lowering additive incorporatedinto it, and particles diffusion bonded to the outer surface.
 2. Anorthopaedic implant comprising a body having an outer surface andparticles sintered to the outer surface, the implant having nitrogendiffused into it.
 3. An orthopaedic implant comprising a body with anouter surface, the body comprising cobalt-chromium-molybdenum alloy, andparticles comprising cobalt-chromium-molybdenum alloy sintered onto theouter surface, the fatigue strength of the implant being greater than orequal to 60 ksi.
 4. The implant of claim 3 wherein the outer surfaceincludes a melting point lowering additive incorporated into it.
 5. Theimplant of claim 4 further wherein the implant comprising the body andbonded particles has nitrogen diffused into it.
 6. The implant of claim5 further wherein the implant is gas quenched to room temperature orlower and aged at a temperature ranging from 800 degrees F to 2100degrees F.
 7. The implant of claim 3 wherein the implant comprising thebody and bonded particles has nitrogen diffused into it.
 8. The implantof claim 3 wherein the implant is gas quenched to room temperature orlower and aged at a temperature ranging from 800 degrees F to 2100degrees F.